Patent GB-1461137-A (GB)
Title: Digital modulate/demodulate system
IPC: H04L27/36
Filed: 1974-10-14
Abstract: 1461137 Data transmission; PSK systems MICROWAVE ASSOCIATES Inc 14 Oct 1974 [12 Oct 1973] 44489/74 Heading H4P Clocked NRZ data 180 deg. phase shifts a constant amplitude carrier to create a (2 + n) waveform which is modified by a partial response filter, band width limited, so that in each data clock period the modulated wave cannot swing more than approx. 1/ (1 + n) of the incoming carrier amplitude at filter output; n is an integer and may equal 1. A linear amplifier may follow the filter or in a modification a non-linear amplifier may precede it. A second NRZ signal, which may be derived with the first from a single source in known manner, may be interleaved with the first through the agency of a second, equal frequency carrier mutually phase shifted 90 deg. which is then combined with the first before application to the filter. Modulation.-In one embodiment of the quadrature partial response system (QPQS) incoming data streams 101, 102 alternately derived from a single source 112 are fed to respective suppressed carrier modulators 103, 104 fed by phase shifted waves from common oscillator 107. An attenuated oscillator pilot output 109 may be combined with 103 output to aid carrier recovery at a demodulator, which in combination with 104 output is fed to an optional upward variable frequency changer 125 and through limited band filter 129 and linear amplifier 131 to provide a 9 state i.e. 8 phase position + zero output signal. In a modification (Fig. 5 not shown) 129 may be replaced by separate filters in lines 123, 124. At the demodulator (Fig. 10, not shown) incoming signals may be downwardly frequency shifted and passed through an amplifier to (preferably identical) demodulators (154, 155) also fed by a voltage controlled oscillator at the same frequency as the (suppressed) carrier of the received signal. The oscillator phase is controlled, so that outputs from demodulators (154, 155) correspond with input waveforms 101, 102, and also fed to 3-level converters (166, 167) to provide separate NRZ outputs which may be combined (at 171) if derived originally from a single source. Demodulator outputs are also fed to a clock extraction circuit (181) having its own phase locked oscillator at half clock frequency output from which feeds the converters and (where appropriate) the data combination circuit. At the transmitter, pilot 109 is in phase with the carrier for waveform 101 hence any DC output from demodulator (154) is representative of phase error between received pilot and oscillator (157) hence through low pass filter (175) and amplifier (176) this adjusts the phase of (157) to a low value. An alternative method of carrier recovery eliminating the need of a pilot signal is envisaged. Where the demodulator circuit is first activated the oscillator frequency may not be within the phase locked loop capture range hence a sweep voltage generator (178) is provided to sweep the oscillator frequency to all possible frequencies. When phase lock is achieved, demodulator (154) d.c. output voltage is driven to a null and demodulator (155) pilot d.c. output voltage is maximized, the appearance of which indicates existence of phase lock condition. This is sent to a phase lock circuit (179) which stops generator (178). Should the phase lock condition be lost such input voltage disappears and activates (178). The 3 level to NRZ converters are described with reference to 12A, 12B (not shown). The arrangement is stated to facilitate transmission of a 01 data stream e.g. 24 channels, over a video (television) channel (see Figs. 11A, 11B not shown), incorporated in a microwave relay system giving increased PCM telephone channels over a given band relative to existing systems. Unused portions of video channels may also be used as subcarriers above video frequencies or frequency division system. To prevent transmission of a train of 1's or 0'o incoming data may be scrambled, e.g. pseudsrandom wise, which also facilitates recovery of clock signals. It is also stated that the invention can be incorporated in systems using other methods of comprising amplitude phase modulation.